Shrouds employed in gas turbines surround and in part define the hot gas path through the turbines. Systems for cooling the shrouds, particularly those directly surrounding the rotating parts, i.e., the gas turbine buckets or blades, in the hot gas path of the gas turbine are oftentimes necessary in gas turbines to reduce the temperature of the surrounding shrouds. Shrouds are typically characterized by a plurality of circumferentially extending shroud segments arranged about the hot gas path with each segment including discrete inner and outer shroud bodies. Conventionally, there are two or three inner shroud bodies for each outer shroud body, with the outer shrouds being secured by dovetail-type connections to the frame of the turbine and the inner shroud bodies being secured by similar dovetail connections to the outer shroud bodies.
The inner shroud body includes a wall which in part defines the hot gas path and which must be cooled, for example, with cooling air from the compressor discharge of the turbine. In prior designs, an impingement plate has been provided in the outer shroud body for receiving the cooling air and directing the cooling air through apertures in the plate for impingement cooling of the inner shroud body wall. This arrangement is not optimum from the standpoint of efficient cooling and requires substantial cooling flow. More particularly, the impingement plate mounted on the outer shroud body in this conventional design is spaced a substantial distance from the wall being cooled by the impingement air flow through the apertures of the plate. The inner shroud body has axially extending reinforcing or structural ribs projecting radially outwardly from the wall being cooled, previously believed to necessitate the location of the impingement plate mounted to the outer shroud a substantial distance from that wall. With this arrangement, cooling efficiency is lost as the impingement cooling air flows over this very substantial distance before impacting and cooling the inner shroud wall. Further, by locating the impingement plate in the outer shroud body, the impingement cooling air sees secondary leakage paths prior to passing through the impingement plate apertures, which causes further inefficiencies in cooling and requires additional cooling flow. Thus, there is a need for an impingement cooling system which will substantially reduce these cooling inefficiencies, eliminate leakage paths and substantially reduce the impingement flow distance between the impingement plate and the inner shroud body wall being cooled by the impingement cooling air flow.